1. Field of Invention
Molding of thermally curable organic plastic materials.
2. Description of Prior Art
For purposes of illustration, the present invention is described herein as applied to tennis ball cores of elastomeric material such, for example, as conventional natural or synthetic rubber compositions, its application to other hollow articles, whether of spherical or other shape, and to other elastomeric materials and compositions, being apparent from the illustrative example. It is difficult, by prior methods, to mold seamless hollow spherical articles to precise shapes and dimensions, both internally and externally, while providing walls of uniform thickness and quality throughout and a joint comparable to the strength of the remainder of the article. Generally, such articles are molded as mating halves which subsequently are joined together along a diametral plane.
One of the earlier methods is to mold and fully vulcanize two halves of the article separately and, after full vulcanization and cooling of the halves, to cement them together to form the finished article by applying cementitious material to the exposed edge surfaces of the halves. This method is the one most commonly used at present despite the fact that bonding the joint by extraneous cementitious material renders the juncture non-homogeneous, and weak compared to the walls.
In later methods, such as disclosed in U.S. letters Pat. No. 1,575,388 to F. T. Roberts, issued Mar. 22, 1926, devised to provide a fused and improved edge bond, the mating halves of an article are formed by molding raw material to shape about a core. The mold is then opened, the core removed, and all excess raw material is removed for immediate reuse without any reconditioning in the next production cycle. While the molds are open a gassing agent is introduced into the raw molded halves. The mold is then closed to press the raw halves together to form the assembled raw article and the raw articles is then vulcanized. During the initial stage of heating, the gassing agent evolves sufficient gaseous pressure within the raw article to assure that the material maintains its position and shape in the mold cavity and remains in firm contact with the cavity walls during vulcanization. This later method has some advantages. For example, the molding and removal of the core are effected while the material remains in its raw condition so that the excess raw material surrounding the mold cavity can be removed, when the mold is opened and the core is removed, and immediately reused in the next succeeding molding operation. It had disadvantages in that generally beads or flash of substantial amount are formed at the seam or juncture of the two halves. Further, the seam tends to be weak due to the fact that the raw material of the halves cannot transfer substantial bonding pressure to the mating edges, upon closure of the mold after removal of the core, and cannot maintain substantial pressure on the edges during vulcanization.
Another disadvantage is that the gassing agent necessarily develops pressure for holding the raw rubber in contact with the mold walls during initial vulcanization subsequent to removal of the core. If the mold is opened and the vulcanized articles are exposed or removed from the mold while still hot, the internal gaseous pressure expands the balls or articles unpredictably so that they are out of round and otherwise distorted. This can be overcome only the time consuming step of cooling the articles in the mold sufficiently to cool the gases therein while the articles remain in the closed mold, or by forming a perforation in each article sufficient to vent and permit the excape of the warm expanded gas and relief of the internal pressure before or immediately upon opening of the mold.
This method and similar earlier methods are not in general use today because it is economically impractical to obtain properly shaped products with properly fused edges consistently. The difficulties reside first in the complexities involved in the internal pressurizing and depressurizing. Second, to avoid the "skin effect," the raw elastomer must be exposed initially to very moderate, substantially non-vulcanizing temperatures, which, in turn, makes it necessary in each cycle to cool the mold from the optimum vulcanizing temperature to a substantially non-vulcanizing temperature, and then raise the mold temperature from substantially non-vulcanizing temperature up to optimum vulcanizing temperature and hold the latter temperature until the cycle is completed. This complicates the equipment required and extends unduly the process time.
The inventions disclosed in my copending applications provide solutions to many of these problems by forming on the two halves, respectively, mateable aligned edged which are shaped and arranged to extend a preselected small distance beyond the parting surface of the two mold plates so that after the core is removed, better contact between these edges upon reclosure of the mold is assured. While the mold remains closed with the core therein, the vulcanization reaction is initiated and caused to progress to a generally uniform degree throughout the charge such that the halves of the article in the mold cavity, including the mateable edges, are sufficiently firm to retain their configuration and remain in contact with the cavity walls without the need for internal gas producing agents after removal of the core and reclosure of the mold. After the removal of the core, the mold is closed and vulcanization of all portions is carried to the optimum selected degree. This uniform degree of vulcanization is less than that which the material is to have in the finished product, which latter is referred to herein as the "optimum vulcanization" and which is about 95 percent of the ultimate vulcanization of which the material is capable.
This prior method is a compromise between the desirability of having the parts fully vulcanized before removal of the core and having the surface strata at and adjacent the mateable edges substantially free from skin effect and at a sufficiently low degree of vulcanization so that, upon removal of the core and reclosure of the mold, these strata can fuse and bond together properly into a homogeneous juncture upon completion of the vulcanization stage of the remainder of the halves of the article in the mold cavity. This generally requires that the initial vulcanization before core removal be below 250.degree. F., much less than the optimum vulcanizing temperature.